When using a portable power saw there is a risk of so called kickback accidents, i.e. the saw sword with its saw chain is violently swung upwards so that it hits the saw operator. This type of accident often seriously injures the saw operator's head, arms or upper body.
In order to reduce the risk of kickback accidents, the power saw is often provided with a chain braking device which is actuated as a result of a kickback. The chain braking device is usually actuated by a so called kickback guard. The construction of such kickback guards are based upon the observation that the saw operator's front hand will fail to maintain its grip against the front handle of the power saw during a kickback. Therefore, the kickback guard consists of a safety handle or safety "glove" being pivotably mounted in front of the the front handle and intended to catch the front hand if it will loose its grip due to a kickback. The hand thereby impacts the kickback guard with a force which will be transmitted to a brake means, for instance a brake band, which will brake the coupling drum and thereby brake the saw chain. In some power saws the motion of the kickback guard is arranged also to release the drive motor of the saw from the coupling drum.
However, it has turned out that the saw operator's front hand does not always hit the kickback guard with sufficient force in order to actuate the chain braking device.
In order to solve this problem it has been suggested to provide an automatic saw chain braking device which is actuated as a response to a detection of a predetermined angle of kickback, i.e. without having to affect the kickback guard. These automatic chain braking devices are, however, rather complicated and expensive, and they cannot be mounted on existing power saws.